1. Field of the Invention
The present invention relates an uninterruptible power supply, and more particularly, to an uninterruptible power supply which provides an sine wave AC output and has an energy recycle function to recharge the internal batteries.
2. Description of Related Art
The uninterruptible power supply (hereinafter abbreviated as UPS) refers to an AC power supply which is switched on to provide AC power at the instant that utility power drops below specified limits or is shut down, wherein an off-line UPS simply filters and passes the utility power to a load when utility power is normal. FIG. 4 shows a structure of an off-line UPS. When the utility power is abnormal (such as the utility power voltage too high or too low), the UPS converts the voltage of its battery (41) to a relatively high DC voltage by a DC/DC converter (42). The relatively high DC voltage is then converted to an AC voltage by alternatively switching on two pairs of switches in the inverter (43). This AC voltage is actually a square-wave AC voltage, as shown at the output of the DC/DC converter (42), which may damage inductive loads such as transformers and motors if applied thereto. Therefore, a higher grade UPS usually has a sine wave AC voltage output. However, it requires very complicated circuitry for the UPS to provide a sine wave AC voltage output and more energy is consumed in the circuit. Thus, there is a need for the above UPS to be improved.